According to current beliefs, thymus-derived lymphocytes (T-cells) are responsible for tumor surveillance and graft rejection. It is thus important to learn how the activities of T-cells are controlled, so that desirable modifications of these activities can be contemplated in the human subject. This study proposes to examine the development of thymus derived lymphocytes in the mouse, correlating changes in the antigenic surface features theta, TL, and an antigen of mature T-cells, MT, with changes in sensitivity to corticosteroids in vivo and with specific T-cell activity in antibody synthesis, and response to the plant mitogens, PHA and Con A. There is very recent evidence that a subpopulation of T-cells may have a suppressor role under certain circumstances; this subpopulation will be searched for by separating T- cells on the basis of surface antigens corticosteroid sensitivaty, effect to adult thymectomy, effect of antilymphocyte serum, anatomical location and ability to recirculate. T-cell populations so separated will be combined to identify synergistic or suppressor effects in the test systems described. It is hoped to determine whether active and suppressor T-cell populations can be individually manipulated in the intact animal so that T-cell responses can be amplified or suppressed at will.